Raising decking

Hi, after some advice again...

I have a 1.5 x 2m section of decking that is currently sitting on some slabs at ground level. I've decided to raise this to approx 4-5ft off the ground, to allow some storage underneath and give the kids a nice high seat with slide coming off it.

Question is.. whats the best way to ensure that my uprights are in the right place to fit under the existing decking? And if I want it to be 4 ft above ground, how deep do the support posts need to go? Any other advice will be gratefully accepted as well!!

Thanks!